Feedback-cancelling electro-acoustic transducer apparatus

ABSTRACT

Electro acoustic transducer apparatus having a loudspeaker and one or two microphones includes positional and/or electric arrangements for reducing or cancelling feedback from the loudspeaker. Also, a microphone apparatus has positional and structural features for reducing or cancelling echo.

United States Patent Gabr 1 1 Nov. 25, 1975 [54] FEEDBACK-CANCELLING2,208 l6O 7/1940 McCreary 179/1 FS ELECTRO-ACOUSTIC TRANSDUCER Eri 1 B181 i! 1 1 1 1 v r APPARATUS 3,588,352 6/1971 Yamawaki 1 l H 179/1 FS[75] Inventor: Saar! Zaghloul Mohamed Gabr, 3,588,360 6/1971 Knox 1179/] HF C m -b E l d 3,784,747 1/1974 Berkley er a]. 179/1 HF [73]Assignee: A.R.D. Anstalt, Vaduz,

Liechtenstein Primary ExaminerTh0mas W. Brown [22] Fikd: Dec 17, 1973Attorney, Agent, or Firm-Young & Thompson [21] Appl. No; 425,527

[30] Foreign Application Priority Data l57l ABSTRACT Unlted Klngdomw'58174/72 Electro acoustic transducer apparatus having a loud- W73 UmtedKmgdom 1071/73 speaker and one or two microphones includes posi' I Itional and/0r electric arrangements for reducing or 79/1 l79/l l79/8l Bcancelling feedback from the loudspeaker Also, a mi- 2 l 1/60 crophoneapparatus has positional and structural feal Bl of Search 1 M B turesfor reducing or cancelling echo [56] References Cited UNITED STATESPATENTS 3 Claims, 34 Drawing Flgures 2,177,769 10/1939 Erickson 179/1 FSUS. Patent N0v.25, 1975 Sheet10f4 3,922,488

M I61 FIG.2A FIGZB FIG. 2H FIG. 21 5762K U.S. Patent Nov. 25, 1975Sheet2of4 3,922,488

U.S. Patent Nov. 25, 1975 Sheet 3 of 4 GOOD FIG. I4

U.S. Patent Nov. 25, 1975 Sheet4of4 3,922,488

abcacb FIG. 75A

FEEDBACK-CANCELLING ELECTRO-ACOUSTIC TRANSDUCER APPARATUS The inventionrelates to duplex or two-way communication systems. particularlyloudspeaking systems.

All such known systems suffer from the effects of acoustic feedback,that is, from the effects of sound from the loudspeaker reaching themicrophone, and from the effects of acoustic echoes and ambient noise.The intelligibility of communication is diminished by both effects.

Acoustic feedback, that is the undesired transmission of sound from theloudspeaker to the microphone, whereby the electrical output signal ofthe microphone includes an unwanted component corresponding to andcaused by the loudspeaker sound reproduction, takes place mainly becauseof the transmission of loudspeaker sound reproduction to the microphonethrough one or more of three channels. One channel is the airborntransmission of sound from the loudspeaker directly to the microphonediaphragm. The second channel comprises loudspeaker sound reproductionreaching the microphone through mechanical vibration of means or mediaother than air, for example the loudspeaker and microphone housing meansand associated fixed structures. The third channel is the transmissionof loudspeaker sound reproduction to the microphone by vibration of themicrophone housing due to airborn sound (microphony).

Echo and ambient noise reach the microphone diaphragm through the airand by microphony.

It is the object of the invention to eliminate or at least reduce bothacoustic feedback and acoustic echo and ambient noise to a point suchthat a satisfactory signal to noise ratio is obtained whereby the stablesound levels and the intelligibility required for a duplex, twoway,loudspeaking telephone conversation is achieved without the need forvoice operated switching.

In accordance with the invention, this objective is achieved in threeways, acoustically, electrically, and electroacoustically. In all casesthe acoustic measures provide for the basic high signal to noise rationeeded. The microphone means is designed and positioned in combinationwith the loudspeaker so as to discriminate between the loudspeaker soundreproduction and the wanted speech or other sound input desired fortransmission, which represents information to be communicated. Theyprovide for the prevention by cancellation or at least the reduction ofthe undesired acoustic feedback noise and acoustic echoes, whereby theelectrical output of the apparatus acquires the required signal to noiseratio which allows satisfactory stable loudspeaking duplexcommunication. Further improvement in the overall signal to noise ratiois achieved if necessary by using non-linear amplifiers and componentsand/or automatic gain control operated by and dependent on the noiselevel, or arrangements giving similar effects. The acoustic measuresadopted in accordance with the invention consist in arranging themicrophone or microphone means in such a position with relation to theloudspeaker or loudspeakers that sound from the latter falls equally ontwo sides of the diaphragm of the microphone means. This can be readilyeffected as far as directly transmitted sound as well as acoustic echoare concerned, but is less readily controlled in respect of reflectedsound, due to objects not forming a part of the equipment, the positionof which is variable or unknown. The acoustic measures of the inventionlikewise provides constructions for the cabinet or housing means,including sub-housings for the individual transducers. which effectivelyreduce or eliminate microphony, and sound transmission through mediaother than air. The effects of sound from uncontrollable local sources,and of loudspeaker sound reflected from likewise uncontrollable objects,cannot be effectively dealt with in either of these ways. To deal withsuch sources of unwanted sound components in the microphone output, theinvention provides electrical circuit arrangements, to reduce theeffects. The electro-acoustic meansures of the invention compriseelectrical circuit arrangements in combination with complexelectroacoustic transducer assemblies. The function of the combinationof circuitry and transducers is firstly to discriminate between theelectrical output signals of the microphone caused by the loudspeakersound reproduction and the speech or other signal desired fortransmission. Secondly. the combination functions to deal with each andto arrange for the cancellation or reduction of the undesired signalscaused by and corre sponding to the loudspeaker sound reproduction andechoes so that the final electrical output signal of the combinedmicrophone means and electrical circuits acquires the high, satisfactoryratio of signal to noise, to allow the stable reproduction oftherequired sound levels from a duplex loudspeaking communication or othersound reproduction system.

In addition to the acoustic measures, the cancellation of the undesirednoise can be achieved by simple electrical circuits to allow for theaddition and subtraction of the outputs of the microphone means, or bycomplex arrangements for summing the signals resulting from the additionand subtraction and also by arranging for phase and amplitude adjustmentif necessary to achieve the required cancellation of noise. It is ofcourse within the invention to apply one, more than one, or all of thesemeasures as is deemed appropriate in any particular piece of equipment.

The invention will be more readily understood from the followingillustrative description and accompanying drawings, in which:

FIGS. 1A, 1B, 1C and ID are somewhat schematic showings of varioussuitable arrangements of microphones and loudspeakers relative to eachother;

FIGS. 2A-2K are arrangements similar to FIGS. lA-lD, but in which themicrophones receive sound directly from the loudspeaker;

FIG. 3 is a circuit diagram of apparatus according to the presentinvention;

FIGS. 4 and 5 are views similar to FIG. 3 but showing other embodimentsof the invention;

FIGS. 611 are views of suitable microphone arrangements for use in thepresent invention;

FIG. 12 is a view of a microphone diaphragm and FIG. 13 is a view ofamicrophone baffle for use in the present invention; FIG. 14 is across-sectional view of a modified housing for use with the presentinvention;

FIGS. 15A and 15B are fragmentary crosssectional views of tubularembodiments of housing;

FIG. 16 is a fragmentary cross-sectional view of a diaphragm mounting;and

FIGS. l72l are vector diagrams of signal and noise components involvedin the present invention.

According to a first aspect of the invention illustrateddiagrammatically in FIGS. 1A, 1B, 1C and ID, a microphone M andloudspeaker L are so relatively located 3 that both sides of themicrophone diaphragm receive sound directly from the loudspeaker. Theresultant effect of this incident sound will be ideally zero. so thatthere is no electrical output from the microphone due to this sound. Themicrophone and loudspeaker must be relatively close together. preferablythey should be in close proximity within a housing means whichadvantageously comprises sub-housings, one for each transducer, suitablyconnected together in a manner or by means whereby sound transmissionbetween the sub housings is eliminated or nearly eliminated. The housingmeans are shown schematically in or omitted from the drawings. A varietyof relative positions are possible, of which some only are shown in theaccompanying drawings. In particular, the microphone can be ar' rangedwith its own diaphragm in any plane containing the loudspeaker axis. Thearrangement of FIG. 1A is found to be particularly advantageous, it isthought because the sound wavefront radiated by the loudspeaker is notbroken up or impeded by the rim of the microphone and associatedhousing. The rim and housing should preferably in any event be generallysmall and thin in particular to afford better cancellation.Alternatively, the microphone and loudspeaker diaphragms can be coplanarand in adjacency. The arrangement must be such that the loudspeakersound reaches the two sides of the microphone diaphragm directly orsubstantially directly, that is without reflection or undue obstruction.In particular, the housing means should not cause undue reflected soundfrom the loudspeaker to fall on either side of the diaphragm. One orboth sides of the microphone and/or the loudspeaker can however becovered with a conventional grill or fabric.

It will be understood that these configurations of loudspeaker andmicrophone can be used in any position; these do not in themselves doanything to counteract acoustic feedback due to reflected sound from theloudspeaker, and it is accordingly an advantage to have the loudspeakerdiaphragm substantially horizontal to provide for acoustic echoreduction or cancellation. At least the majority of any reflectedloudspeaker sound which reaches the microphone is then due to reflectionfrom the ceiling of the room in which the apparatus is used. Thedistance of the ceiling will normally result in considerable atenuationof this reflected sound, which is moreover a constant factor in any oneroom, which facilitates compensation.

Whilst the arrangements shown comprise only a single microphone and asingle loudspeaker, it will of course be appreciated that the actualtrariducer units employed as loudspeaker and microphone can beduplicated or used in any desired numbers. Thus a loudspeaker may haveone or more additional microphones M positioned similarly with respectto the loudspeaker as the microphone M. For example four microphones canbe located in planes containing the loudspeaker diaphragm axis andangularly spaced at right angles around the axis. The microphones couldthus receive speech from speakers grouped around the apparatus as at aconference.

It is also within the invention to arrange a pair of microphones in aposition with relation to the loudspeaker such that each microphonediaphragm receives on one side thereof equal or substantially equalsound signals directly from the loudspeaker. Some possible arrangementsare diagrammatically shown in FIGS. 2A 2K. If the arrangement is suchthat the signals received are in phase. the outputs of the twomicrophones must be rewrsed. so that the electric signal components dueto the loudspeaker sound cancel. and are thus absent from the commonmicrophone output. Ifthe arrangement is such that the sound signals areout of phase, the microphone outputs are simply added to provide thecommon electrical output. It will he evident that the or each microphoneof any of the arrange ments of FIG. 1 can be replaced by a pair ofmicrophones arranged with their diaphragms parallel and spaced somedistance apart. and either separated acoustically by a baffle, or insubstantially direct acoustic coupling to realize the present aspect ofthe invention. It is however also possible to arrange the twomicrophones in other ways. As with the arrangements of FIGS. 1A ID, itis of course readily possible to add one or more additional pairs ofmicrophones. each pair being appropriately positioned relative to theloudspeaker. Again. more than one loudspeaker can be employed, providedthe correct relative positions are preserved.

In order to ensure that the loudspeaker sound signals reaching themicrophones are as nearly as possible identical in all respects, thedistance between the axes of the microphone diaphragms should be keptsmall compared to the acoustic wavelength of the signals. Preferably,the greatest distance involved should not exceed 14; of the wavelengthof the highest frequency to be handled by the apparatus, which may beregarded as 3.000 Hz where only speech is being transmitted.

It is within the invention to arrange for the operation of theloudspeaker or loudspeakers and the microphone or microphonesalternately or on a time division basis for example by direct switchingor by switching the energisation of associated amplifiers. The frequencyof alternation should be high enough not to cause interference with thereproduction. With this method, the distance between the loudspeaker andmicrophone diaphragms must be kept small to avoid an undesirable timelag in transmission. Otherwise, it is advantageous for this distance tobe relatively large. to allow the wave front of loudspeaker sound to bein phase across the microphone diaphragms.

It will be understood that the arrangements of FIGS. 2A 2K will includehousing means preferably of the kind described with reference to FIGS.1A ll). The measures so far described provide a substantial improvementover known arrangements, which have dealt only with acoustic feedbackdue to transmitted sounds reaching the microphone diaphragm ordiaphragms via the air directly, that is, direct air-born sounds fromthe loudspeaker.

In any of the two-microphone arrangements of the invention, themicrophones can be constituted by the so-called button or midgetmicrophones, the inlet nozzles or tubular apertures of which will asregards posi tion replace the diaphragms illustrated. The use of thesebutton microphones provides the closest possible spacing between themicrophone sound input positions so that the phase difference betweenthe microphone outputs is minimised.

FIG. 3 shows an acoustic feedback-free transducer unit F providing ahigh signal to noise ratio and including two microphones M. Within aphase adjustment unit U incorporating electric circuit arrangements forfurther improving the signal to noise ratio the outputs of the twomicrophones M are applied both to an additivc circuit element A and to asubtractive circuit element 5. for example a transformer or adifferential amplifier. The additive and subtractive outputs are thentaken through phase and amplitude control means P and summed eithersimply or repeatedly, preferably in a further amplifier 0, to providethe electrical output signal to be transmitted. The treatment of themicrophone outputs is described in detail below with reference to FIGS.17 21. The circuitry can include the circuit elements necessary toeffect appropriate phase and/or amplitude adjustment. Each of thevarious circuit elements is preferably adjustable and may be arrangedeither for presetting or for control from outside the unit. The unit Fincludes a loudspeaker L, the relative positioning of the microphones Mand the loudspeaker shown diagrammatically being of course only one ofvarious possible arrangements.

It will be appreciated that this aspect of the invention consists inisolating an electric signal produced by un wanted sound falling by anychannel on a microphone or microphones, and subtracting this electricsignal from a microphone output containing both an unwanted soundcomponent and also a wanted sound component. The output of the unit Uand the input to the loudspeaker L and its associated amplifier areconnected through a hybrid H if required, to a line and thereby to likeapparatus at a remote station. A conventional automatic gain controlcircuit G, e.g. in the form of two conventional units, may be includedas shown to control a loudspeaker amplifier K and the output amplifier Oto improve efficiency, but this can often be omitted if desired.

Two other circuit arrangements for the electrical cancellation offeedback are illustrated respectively in FIGS. 4 and 5. These can alsoinclude automatic gain control arrangements as described with referenceto FIG. 3 if desired. In each arrangement, an acoustic feedback freetransducer unit F of any desired kind comprises housing means mountingloudspeaker means represented by a single loudspeaker L and microphonemeans represented by a pair of microphones M.

Referring now to FIG. 4 the loudspeaker L has two speech coils l2, 14.The microphone outputs and the loudspeaker reproduction coil 12 areconnected through respective amplifiers l5, 16 to a hybrid l8 and thusto a line terminal 19. The hybrid 18 enables the station to be connectedto a two-wire transmission system but may be omitted if a three or fourline transmission system is employed.

In FIG. 4, part of the output of the transmission amplifier is taken ona line 20 through an adjustable resistor 2I to the coil 14, thetransmission signal cancella tion coil, of the loudspeaker. The line 20also extends through a phase adjuster 22 which may instead be in eitherof the alternative positions indicated at 22' and 22". When there is asound input to the microphone, part of the resultant signal reaches eachof the coils I2, 14 through the hybrid or through coupling between thelines caused by the other station or stations. In order to remove this apart of the signal is fed through the line 20 and the circuit components21 and 22 such that these signals are equal and 180 out of phase.

In FIG. 5, an amplifier 24, preferably of variable gain, is included inthe line 20 together with an adjustable phase shifter 25 which caninstead be placed at the position indicated by 25'. The operation of thecircuit is essentially the same as that of FIG. 4 except that theamplifier 24 is provided to ensure that the signal applied to the coil14 is of adequate strength. Also, the loudspeaker 1] has only a singlespeech coil 26 connected with the secondary winding 28 of a transformerhaving primary windings 29, 30 performing the functions of the coils l2,14 of the loudspeaker of FIG. 4.

It will be evident that the circuits of FIGS. 4 and 5, operating betweenthe hybrid and the transducer unit, can be employed in Ioudspeakingtelephones and can be adapted to systems using two pairs of wires fortrans' mission as for intercommunication or public address systems.Although cancellation takes place at the loudspeaker in both circuits,cancellation in a like manner at the microphone, additionally orinstead, can be provided. Moreover, the circuit arrangement ofFIG. 3 canbe employed in combination with the arrangements of FIGS. 4 and 5whenever the transducer unit F includes two microphones.

Whilst the feedback free transducer unit of the invention can includeloudspeakers and microphones of any suitable construction, microphonesof preferred con struction are shown in FIG. 6 and 7.

Referring to FIG. 6, an electro-acoustic transducer for use as amicrophone or loudspeaker consists of a diaphragm 5], a speech coil 53and a frame 54 for rigidly supporting a magnet 52. The frame 54 exposesa major area of the adjacent side of the diaphragm. The diaphragm 1therefore may be exposed to sound waves on both sides. Such a microphonehas bi-directional characteristics and therefore is particularly usefulfor acoustic suppression of the feedback. Ideally, the microphone shouldhave symmetrical bi-directional characteristics and to this end theplanar diaphragm 51 is used as distinct from the conventional conicaldiaphragm. As the magnet, coil and frame structure are on one side onlyof the diaphragm, the microphone is mounted, in accordance with theinvention, in a housing providing dummy, elements (not shown) on theother side the dummy elements being of at least approximately the sameshape and location. The apparatys comprisisng the microphone and thehousing thus has the desired symmetry. The diaphragm 51 can if requiredbe corrugated whilst retaining its generally planar form.

The operative sensitivity curves for such a transducer depends on themechanical suspension or electrical sensitivity control of thediaphragm. As the former cannot be readily adjusted, it is convenient toprovide for electrical sensitivity control by biasing to obtain therequired performance. The microphone can thus be provided with biasingcoils in addition to the normal speech coil.

An electro-acoustic transducer apparatus which is itself physically andelectrically symmetrical will avoid the need to provide dummy elementsin its housing, and is shown in FIG. 7. This transducer corresponds inits general form on one side of the diaphragm to the transducer of FIG.6. However the speech coil, magnet and frame structure is repeatedsymmetrically on the other side, the parts on this side being markedwith primed reference numerals corresponding to those on the one side. Abiasing coil can be provided to adjust the suspension of the diaphragm,as previously described.

FIG. 8 shows a microphone unit having a pair of like electro-acoustictransducers M mounted one on either side of a baffle board 66. Thetransducers can be of conventional moving coil construction, with theaxes aligned.

The conical diaphragms of the transducers face one another and arespaced apart at their edges only by the edge mounting boards (which maybe omitted) and the thickness of the board 66. The transducers are eachmounted in an open ended loading chamber or baffle box 72 having onewall constituted by the board 66 and adjacent four sides 74 at rightangles to the board and surrounding the transducer. Slits 76 can beprovided in the walls 74 to ensure a communication between the interiorof the box and outside. The walls 74 should not extend so far axially ofthe transducers as to impose directional characteristics on these,unless such characteristics are desired. The mounting arrangements thuscomprise a housing providing for each transducer something approachingan infinite baffle.

Circuit arrangements associated with the transducers can comprise meansfor adjusting the phase and/or amplitude of the output of thetransducers as previously explained. As shown, a potentiometer isconnected across the coil of each, the tapping contacts of thepotentiometers providing the output of the unit across terminals 75.

In use, the microphone unit will be effectively echocancelling becausereflected sound will in general fall in substantially equal intensitieson the two transducers and will consequently cancel out in theelectrical output signal from the unit. Desired sound, for example, froma speaker holding the unit so that he directly faces one of thetransducers will be fully reproduced in the electrical output, as alsowill speech from a second speaker opposite the first. The potentiometerspermit adjustment of the unit to compensate for departures thereof fromexact symmetry, electrical or structural, about the central plane of thebaffle board 66.

In the embodiment of the invention, shown in FIG. 9, one or morefeatures of which may be interchanged with those of the unit shown inFIG. 8 as appropriate, two like transducers M are again employed. Thesetransducers have substantially planar diaphragms are previouslydescribed. The transducers M are mounted coaxially on a baffle board 76which is apertured either over substantially the whole area of thediaphragms, or over part of this area. In this arrngement, themicrophone orientation can be reversed, to expose the full area of thediaphragms to incoming sound. The transducers M are connected intoappropriate circuitry, for example, as shown in FIG. 8 or as previouslyexplained with reference to FIG. 3.

In use the microphone unit of FIG. 9 as so far described will be seen tohave the same echo-cancelling properties as that of FIG. 8. The acousticcoupling of the two transducers however causes both to respond to anincoming desired sound along the common axis from one side of the unit,so that the output and thus the ratio of this to any uneancelled noiseis substantially doubled.

The microphone unit of FIG. 9 additionally incorporates two furtherelectro-acoustic transducers M for use as loudspeakers, so that the unitbecomes a device for two-way communications, usable for example as aconventional or loudspeaking telephone. The transducers M are mounted soas to preserve the symmetry of the device and with their axes at rightangles or nearly at right angles to the common axis of the transducer M.Such an arrangement provides for acoustic cancellation of feedbackwithout loss of the echocancelling properties of the microphone unitcomprising the transducers M.

When a baffle is provided between the two diaphragms, the otherdiaphragm of course remains unaffected by this sound and the output fromthe diaphragm receiving this wanted signal is substantially unchanged bythe sound travelling parallel to the diaphragm. If there is no bafflebetween the two diaphragms, the second diaphragm, which does notdirectly receive the incoming wanted sound, moves in sympathy with thefirst mentioned diaphragm because of the vibrations of the firstmentioned diaphragm conveyed through the air between them.

The microphone unit shown in FIGS. 10 and 11 has a single transducerhaving a generally planar diaphragm and a pair of speech coils, one oneither side, co-operating with separate magnets. It is however to beunderstood that a pair of microphones can be used instead, and thatappropriate features of the units of FIGS. 8 and 9 will then be includedin the present unit. In the present embodiment, apertures, which may beselectively adjustable in effective area, are provided between theloading chambers associated with the respective sides of the transducerAs shown in FIG. 10, one form of such an aperture comprises an annularaperture 34 around the transducer and a baffle 35, which functions as aby-pass or short circuiting aperture. Thus the diaphragm suspension canbe clamped at its outer edge between a pair of rings 86 from each ofwhich radial arms 88 extend to support one of the magnets. Two bolts orother fasteners extend outwardly from the rings 86 at diametricallyopposed points and are the sole means of connection with the baffle 85,which has an aperture of greater diameter than the rings. The resultingannular aperture 84 effects a degree of equalization of sound vibrationson opposed sides of the baffle. The bolts can extend through rubber orplastics grommets 91 so that the transducer is effectively isolated fromsound vibrations transmitted through the housing.

Where the baffle is a square as in the unit of FIGS. 10 and II orrectangular, an aperture 92 in the baffle can be provided at each cornerto minimise the effect of sound reflections at the corners between thewalls of the loading chambers extending from the baffle 85.

Instead of, or as well as, providing such apertures in the baffle, it iswithin the invention to provide them in the actual transducer diaphragmor diaphragms.

The effective cross-sectional area of all the various kinds of aperturesdescribed is preferable made selectively variable, and even reducible tozero, by any suitable form of closure or shutter to permit eitherpre-set adjustment or adjustment in use or both. As shown in FIG. 12,apertures can be arranged for example as a series of holes along aradius or a diameter of a diaphragm I01 and their area renderedadjustable by a blocking means (not shown) rotatable about the axis ofthe transducer. The blocking means can be rotatable disc but needcomprise only a radial or diametral arm to function as required and canbe constituted by a radial arm of the transducer structure by which themagnet is connected to the outer ring from which the diaphragm issuspended.

Alternatively, as shown in FIG. 13, the apertures may comprise holes 102towards the edge of a circular bafflc 104. The effective area of theseholes can be adjusted by rotating of a ring 105, mounted concentricallywith the baffle, which has holes corresponding in shape and position tothe holes 102.

According to a further feature of the present invention, the transduceror transducers of the microphone unit are provided with a sounddiffusing cover to minimise thedirectional effects of noise and theeffects of reflected noise. Such a cover can be constituted by a pieceof acoustic transparent fabric secured over each side of the transducerframe. The diffusion effect can be controlled by controlling theacoustic transparency of the fabric.

The microphone units so far described are inherently bi-directional,being completely symmetrical about the plane of the baffle. The unitscan however be rendered uni-directional, as regards the incoming soundvibrations which are to be transmitted electrically, without loss oftheir noise and echo free properties. This can be achieved as shown inFIG. I4 by modifying the housing means to provide at the outer end ofone of the loading chambers 106 passage means 107 communicating with thechamber at one end and with the desired direction of sound reception atthe other, entry to this chamber of sound from the other direction beingblocked. As the microphone unit is constructed as an elongated tube, thepassage forming means 107 is constituted simply by a tube of largerdiameter with one end closed, the bidirectional unit being mounted withone end extending concentrically into this tube. The other end of theunit functions normally but sound can reach the side of the diaphragmtowards the tube only through the annular passage between the tube andthe outside of the housing of the unit. As the force acting on thediaphragm is inversely proportional to the square of the distance between the source and the diaphragm and since ambient noise and echo canbe regarded as from a distant source, the extra path length representedby the annular passage has no appreciable effect on the ambient noiseand echo cancellation properties of the unit, whilst having appreciableeffect on sound from a relatively near source. The annular passage willof course reduce the noise level as well as the microphony effect of themicrophone housing but will also reduce the sensitivity, as it providesfor the wanted sound to reach both sides of the diaphragm so that adegree of cancellation of this sound also occurs. however, the wantedsound will be from a source which is relatively close to the unit sothat extra path length of the part of this sound reaching the remoterside of the diaphragm will cause a lowering of the intensity of thispart as compared with the part reaching the near side of the diaphragmdirectly, so that an adequate net output is obtained. It will be evidentthat the means forming the passage which converts the bi-directionalunit to a uni directional unit can be constructed in various way otherthan as described.

Although the invention is not limited to the use of any particularmaterial for the housing means, subhousings, housing, baffle, etc..these are preferably built up in a manner similar to that illustrated inFIG.

15 which shows a tubular housing made from a plurality of rings of likeinner and outer diameter arranged in caxial alignment to form a tube inthe manner shown in FIG. A. The rings are made of different materials a,b, 0, each having different sound conductive properties. Thus one ringmay be formed of metal, the next made of glass fibre and the third ofrubber or a hard or soft plastics material. Sound transmitted throughany one ring to the next is very substantially attenuated at theinterface and the whole structure is effectively acoustically "dead" ornonmicrophonous. Instead of repeating the sequence of, say, metal, glassfibre and plastics rings, it is preferred instead to use for the nextthree, rings of the same three materials but in a different order, againas shown in FIG. 15A, for example,

rings of glass fibre, metal then plastics. The next three rings can thenbe arranged in the order metal, plastics and glass fibre. In this way asequence of rings of different materials is obtained, in which each ringhas on either side of it a ring of different material, and the num berof instances in which a sequence of adjacent rings is kept to a minimum.Obviously this aspect of the invention is not confined to the use ofthree materials only, to the materials mentioned above, or to anyparticular sequence in which the rings, or layers of other shapes, arearranged.

It will be appreciated that a tubular microphone housing built up asdescribed will have almost infinite resistance to sound conduction inthe axial direction. Sound conductivity within each ring radially of thehousing will however be no more reduced than if the housing wereconstructed of an integral piece of the material of the ring. In orderto obtain the required high sound attenuation characteristics in theradial direction also, the housing can be formed not simply of axiallyadjacent single rings but of axially adjacent concentric rings,preferably at least three in number and again of different materials, asshown in FIG. 158. Thus the concentric ring setjust described may haveon one side three similarly shaped rings of which the inner one is offibre glass, the middle of metal and the outer of rubber, whilst on theother side the glass fibre is on the outside, the metal ring being theinner ring and with rubber inbetween.

It will be evident that the structure described for a microphone housingcan be adapted readily to housings of any shape, and to housings andsub-housings for other apparatus than the echo-cancelling microphoneunits and transducer apparatus described. Thus flat panels can be formedfrom several layers of different material the layers themselves beingmade up of part of different material.

The various components of the housing described can be connectedtogether in any suitable way, as by adhesive layers, or by snap-fittinginter-connections making use of resilience inherently possessed by someof the materials used.

The structure described can be applied, as well as to the housing, alsoto the mounting and actual structure of the electro-acoustictransducers. Each transducer as shown in FIG. 16 thus may have theplanar diaphragm constituted by a circular metal disc 120, a pair ofspeech coils one on each side, and a frame supporting magnets forco-operation with the coils. The frame consists of two like parts 121which clamp the diaphragm suspension 122 between them. Each frameportion is made of metal or rigid plastics material but the two portionsare at no point directly in contact. Instead, each portion is given acoating of a softer, resilient material 124 such as rubber or a plasticsmaterial. The symmetry of the arrangement is rigidly preserved and soundreflecting obstructions such as boards mounting connecting tags areeliminated as far as possible. The frame is thus a structure of threedifferent materials, that of the frame portions, the coating, and thesuspension.

In a modified form of the transducer, one of the speech coils and theassociated magnet are omitted, the other features described beinghowever retained.

The microphone can be mounted within the tubular housing described aboveby providing the latter with a pair of inwardly projecting flanges,advantageously of the same laminated construction as the housing. the

1 1 outer edge of the frame of the microphone being clamped or otherwisesecured between them.

Throughout the unit, internal sound reflections are reduced oreliminated by the provision of rough, sound absorbent, surfaces. Anymetal surfaces can be embossed or etched and all surfaces may be coveredby flocking or by securement of a suede or other nonreflective fabric. Arubber compound can be applied by dipping or spraying to the variousparts of the unit.

The invention can also be embodied in a telephone hand-set. The hand-setembodying the invention comprises three effectively acousticallyseparated portions, the walls of which may be built up from laminationsin a manner similar to that already described in connection with theecho-cancelling microphone housing. These are a stem, an ear-piece atone end of the stem, and a microphone and its associated housing at theother.

The functioning of the apparatus of FIG. 3 is explained in FIGS. 17 to21. Each of the microphones M produces an output including a wantedcomponent or signal due to sound generated remote from the apparatus,typically the speech of someone using the apparatus. The signals S inthe microphone outputs can be regarded as being effectively equal in allrespects. In addition, each output includes a component due to soundfrom the loudspeaker, which is an unwanted component or noise. The noisecomponents N,, N will differ in phases and amplitude from each other andfrom the signals 8, as shown in FIGS. l7, 18 which represent theseparate microphone outputs.

By the circuits A, S the two microphone outputs are respectively addedand subtracted. The output of circuit A contains the signal Seffectively doubled in amplitude, but the signal S is substantiallylacking from the output of circuit S. As shown in FIGS. 19, 20, thecircuit outputs contain respective noise resultants N N The microphoneoutputs have thus been analysed and a component N has been separated outwhich represents substantially pure noisev Such a pure noise componentcan be regarded in practice as having a substantially fixed relationshipwith the noise components N N and with the additive noise component N Itcan accordingly be produced and treated to be substantially equal inamplitude and opposite in phase, for the purpose of cancellation, to anyone of N N and N In the illustrated apparatus, the noise resultant N isamplified and shifted in phase through in a circuit P and added to orcombined with the output of circuit A as shown in FIG. 21, as in theamplifier O. The output of this amplifier which constitutes a commonmicrophone output is thus free or substantially free of components dueto loudspeaker noise.

I claim:

I. An electro-acoustic transducer apparatus comprising a loudspeaker andmicrophone means having two closely spaced transducers, electroniccircuit means, said circuit means including an additive circuit elementand a substractive circuit element, each of said elements receivingoutput from both of said transducers, said elements providing twooutputs one of which is wanted sound plus noise and the other of whichis only noise, said circuit means comprising also phase adjustment meansto receive said other output and to produce a noise component 180 out ofphase with said other output, said circuit means comprising also meansto combine said one output with said out-of-phase component to cancelnoise from said one output.

2. Apparatus as claimed in claim I, and a housing mounting saidloudspeaker and microphone means together.

3. Apparatus as claimed in claim 1, and an amplifier for the microphonemeans and an amplifier for the loudspeaker, and automatic gain controlmeans for controlling the gain of the amplifiers.

l =0 i i I!

1. An electro-acoustic transducer apparatus comprising a loudspeaker andmicrophone means having two closely spaced transducers, electroniccircuit means, said circuit means including an additive circuit elementand a substractive circuit element, each of said elements receivingoutput from both of said transducers, said elements providing twooutputs one of which is wanted sound plus noise and the other of whichis only noise, said circuit means comprising also phase adjustment meansto receive said other output and to produce a noise component 180* outof phase with said other output, said circuit means comprising alsomeans to combine said one output with said outof-phase component tocancel noise from said one output.
 2. Apparatus as claimed in claim 1,and a housing mounting said loudspeaker and micropHone means together.3. Apparatus as claimed in claim 1, and an amplifier for the microphonemeans and an amplifier for the loudspeaker, and automatic gain controlmeans for controlling the gain of the amplifiers.